The present invention relates generally to microwave filters, and more particularly, to general response dual-mode, dielectric resonator loaded cavity microwave filters and multiplexers for use in transmitters and receivers for satellite and wireless system applications.
1. Field of the Invention
The present invention relates to microwave filters for use in transmitters and receivers designed to meet difficult requirements of small size, low weight, and tolerance to extreme environmental conditions. Filters according to the teachings of the present invention are thus suited to use in mobile, airborne, or satellite and wireless communication systems in which the requirement exists to sharply define a number of relatively narrow frequency bands or channels within a relatively broader portion of the frequency spectrum. Thus, filters designed according to the present invention are especially useful in bandpass configurations which define the many adjacent channels utilized in satellite communication stations for both military and civilian purposes.
Such satellite communication stations have come to be used for a variety of purposes such as meteorological data, gathering, ground surveillance, various-kinds of telecommunication, and the retransmission of commercial television entertainment programs. Since the cost of placing a satellite in orbit is considerable, each satellite must serve as many communication purposes and cover as many frequency channels as possible. Consequently, the ability to realize complex and sophisticated filter functions in compact and lightweight filter units is a significant advance which permits the extension of frequency band coverage without an increase in size or weight. Moreover, these advances are possible without relaxing the stringent requirements which must be met by such communication systems, including the requirement to maintain stable performance over a wide range of temperature.
2. Description of the Prior Art
U.S. Pat. No. 3,205,460 issued to E. W. Seeley et al. discloses a microwave filter formed of rectangular waveguide dimensioned to be below cutoff at the frequencies for which the filter is designed. However. a rectangular slab of dielectric extends from top to bottom of the waveguide at spaced intervals along the midplane line of the waveguide, such that a series of spaced susceptances is produced. Tuning screws were used to permit fine tuning of the filter. However, this patent contains no information concerning how to realize filter functions more complex than a simple iterative bandpass design. In particular, there are no teachings as to how to employ dual mode operation, or as to ways to realize cross-couplings for filter designs that require them.
U.S. Pat. No. 3,475,642 issued to A. Karp et al. discloses a slow-wave structure in which a series of spaced discs of rutile ceramic t .tend along a waveguide. The patent contains no teachings of the advantages of using dual mode operation, and employs single mode operation in the TE.sub.01.sup..sub..delta. mode.
U.S. Pat. No. 3,496,498 issued to T. Kawahashi et al. discloses a microwave filter in which a series of metal rods, each dimensioned to be a quarter wavelength long at the frequencies of interest, are spaced along a waveguide structure to form the filter. The rods may be grooved to vary their electrical length without changing their physical length.
U.S. Pat. No. 4,019,161 issued to Kimura et al. discloses a temperature-compensated dielectric resonator device utilizing single-mode operation in the TE.sub.01.sup..sub..delta. mode.
U.S. Pat. No. 4,027,256 issued to Dixon discloses a wideband ferrite limiter in which a ferrite rod extends axially along the center of a cylindrical dielectric structure and through the centers of a plurality of dielectric resonator discs that are spaced along the resonant structure. The patent contains little of interest relating to realization of microwave filter functions in compact high performance filter units.
U.S. Pat. No. 4,028,652 issued to Wakino et al. discloses a single-mode filter design in which a variety of differently shaped and dimensioned ceramic resolution elements are disclosed and described. The patent does not, however, suggest the use of dual-mode operation of any of the resonant structures.
U.S. Pat. No. 4,142,164 issued to Nishikawa et al. discloses a dielectric resonator utilizing the TE.sub.01.sup..sub..delta. mode. The patent primarily discloses the technique of fine tuning by the application of selected amounts of a synthetic resin which bonds to the ceramic resonator elements to incrementally alter their resonant frequencies. There is no suggestion to use dual-mode operation.
U.S. Pat. No. 4,143,344 issued to Nishikawa et al. discloses a microwave resonant structure that utilizes two modes in its operation. However, the modes utilized, using the nomenclature of this reference, are the H.sub.01.sup..sub..delta. and E.sub.01.sup..sub..delta. modes which have very dissimilar field distributions. At least partly as a consequence of this fact, the reference contains no teachings as to how to control coupling to each of the modes, and therefore does not show how to realize one pole of a filter function with each of the modes. As a result, there would be no way within the teachings of this patent to realize a complex 6-pole response in a filter having only three resonators, as could be done if coupling to each of the modes could be independently controlled.
U.S. Pat. No. 4,184,130 issued to Nishikawa et al., and covers a filter design employing a single mode (TE.sub.01.sup..sub..delta. ) in a resonator which is coupled to a coaxial line by means of a short section of that line which has been made leaky by cutting apertures in the outer conductor.
U.S. Pat. No. 4,197,514 issued to Kasuga et al. discloses a microwave delay equalizer. There is no suggestion as to how to make miniature high performance filters that implement complex filter functions.
In addition to the above prior art which disclose solid, high dielectric constant resonant elements, there is prior art in which unfilled cavity resonators of a variety of configurations are employed, sometimes with dual-mode operation. However, due to the unity dielectric constant of the resonant space, the resultant structures are relatively bulky. The prior art relating to unfilled cavity resonators includes U.S. Pat. No. 3,697898 to Blachier et al., U.S. Pat. No. 3,969,692 to Williams et al., U.S. Pat. No. 4,060,779 to Atia et al., and British Patent No. 1,133,801 to Craven.
The Williams et al. patent discusses dual mode filters utilizing conventional cavity resonators, while the British patent utilizes evanescent modes. However, none of this prior art relating to unfilled cavity resonators contains any suggestion to significantly reduce the volume of the resonant structure by employing resonator element of high dielectric constant as the principal component of the resonator, while enclosing this element within a reduced-dimension cavity which would itself be below cutoff at the frequencies of interest were it not for the included resonator element.
An article by Kobayashi et al. entitled "Resonant Modes of a Dielectric Rod Resonator Short-Circuited at Both Ends by Parallel Conducting Plates", 8099 IEEE Transactions on Microwave Theory and Techniques, vol. MTT-28, No. 10, October 1980, New York details experimental studies of resonant modes in a dielectric rod short-circuited at both ends by conductive plates. The Kobayashi reference does not disclose or suggest structures needed to form a functioning microwave filter. For example, no means are provided for tuning the resonator along each of a pair of orthogonal axes, no input and output means are provided.
An article by Plourde et al. entitled "Microwave Dielectric Resonator Filters Utilizing Ba(2) Ti(9) O(20) Ceramics", 1977, IEEE MTT-S International Microwave Symposium Digest discloses a stripline resonant structure different from the coupled-cavity structure employed by Applicant.
An article by Guillon et al. entitled "Dielectric Resonator Dual Modes Filter", 8030 Electronics Letters, vol. 16, (1980), August, No. 17, discloses a single-cavity filter which is more nearly a laboratory model for investigation of microwave phenomena than a completed filter design. Consequently, the reference fails to disclose the intercavity-coupling structure and other details which would be necessary to realize a successful multi-cavity design such as would be required for a practical six, eight or more pole filter function.
An article by Pfitzenmaier entitled "A Waveguide Multiplexer with Dual-Mode Filters for Satellite Use", 5th European Microwave Conference, Sep. 1-4, 1975, Hamburg, Germany is representative of prior art discussed above relating to unfilled cavity resonators. In common with the patent references mentioned relating to unfilled cavity resonators, the Pfitzenmaier reference lacks any suggestion as to how to reduce the bulk and weight of prior art unfilled cavity resonators.
An article by Mahieu entitled "Low Conversion Losses Up and Down Converters, Using Dielectric Resonators for Application with Millimetric Telecommunication Systems", 6th European Microwave Conference, Sep. 14-17, 1976, Rome, Italy discloses frequency converters and mixers employing dielectric resonators, but does not teach the employment of such elements in filter realizations.
With regard to the most relevant prior art, U.S. Pat. No. 4,489,293 issued to Fiedziuszko et al. and assigned to the assignee of the present invention, discloses filter functions in the form of compact filter units that use composite resonators operating simultaneously in each of two orthogonal resonant modes. Each of the orthogonal resonant modes is tunable independently of the other, such that each can be used to realize a separate pole of a filter function.
The composite resonators comprise resonator elements made of a high dielectric constant solid material and may comprise short cylindrical sections of a ceramic material, together with a surrounding cavity resonator that is dimensioned small enough in comparison to the wavelengths involved that it would be well below cutoff but for the high dielectric constant resonator element within the cavity.
Capacitive probes or inductive irises may be used to provide coupling between several such composite resonators, and also to provide input and output coupling for the filter unit formed of the composite resonators. By suitably positioning the coupling devices with respect to the two orthogonal resonant modes, it is possible to achieve cross-coupling between any desired resonant modes, such that filter functions requiring such couplings can easily be realized.
Independent tuning of the orthogonal resonant modes is achieved by the use of a pair of tuning screws projecting inwardly from the cavity wall along axes that are orthogonal to one another. Microwave resonance along either of these axes is coupled to excite resonance along the other by a mode coupling screw projecting into the cavity along an axis which is at 45.degree. to the orthogonal mode axes.
While the filter disclosed in U.S. Pat. No. 4,197,514 was a significant improvements in the filter art, the present inventors have developed a more generalized filter than is disclosed in this patent that provides for variable input/output coupling and which is readily adaptable to many filter applications.
Thus, it would be advantageous to have a microwave filter which can readily realize complex filter functions involving several or many poles, or cross-couplings between poles, and which has variable input/output coupling. It would also be advantageous to have a plurality of composite resonators, together with microwave coupling arrangement therebetweeti to form a filter capable of realizing a variety of complex filter functions within a compact and lightweight unit, and which have variable input/output coupling.
It would be advantageous to have a composite resonator that causes simultaneous resonance in each of two orthogonal resonant modes, and that may be separately tuned for each of the orthogonal modes. It would also be advantageous to have the ability to perturb the fields in the resonator to control coupling between the two orthogonal resonant modes.